Deformation of Two-Dimensional Amorphous Granular Packings

A microscopic understanding of how amorphous materials deform in response to an imposed disturbance is lacking. In this thesis, the connection between local structure and the observed dynamics is explored experimentally in a disordered granular pillar subjected to a quasi-static deformation. The pillar is composed of a single layer of grains, allowing for easy visualization of all particles throughout the deformation. The addition of a liquid into the system causes capillary bridges form between the grains, making the grains cohesive. The two-dimensionality of the system ensures that the liquid is distributed uniformly throughout the packing, making the cohesive forces between the grains known everywhere. We perform separate experiments to measure these capillarity-induced forces, and we find these measurements to be in excellent agreement with our theoretical model and numerical calculations. In the main experiments presented in this thesis, we explore the quasi-static deformation of granular pillar subjected to uniaxial compression. We find a statistical correlation between the local dynamics, characterized by the deviatoric strain rate, and the local structure, characterized by a new measure, introduced here, akin to a relative free area. This correlation is stronger in the presence of cohesion and indicates that regions that are more (less) well packed relative to their surroundings experience lower (higher) strain rates than their surroundings. The deviatoric strain rate also highlights shear bands within the deforming pillar. These shear bands are transient, moving around as the compression occurs. We have developed a way to identify these extended bands, and we compare the structure within these bands to the structure outside. Preliminary results suggest that these shear bands coincide with paths through the material that tend to have more underpacked regions than other parallel in the vicinity of the shear band

Deformation-Driven Diffusion and Plastic Flow in Two-Dimensional Amorphous Granular Pillars

We report a combined experimental and simulation study of deformation-induced diffusion in compacted two-dimensional amorphous granular pillars, in which thermal fluctuations play negligible role. The pillars, consisting of bidisperse cylindrical acetal plastic particles standing upright on a substrate, are deformed uniaxially and quasistatically by a rigid bar moving at a constant speed. The plastic flow and particle rearrangements in the pillars are characterized by computing the best-fit affine transformation strain and non-affine displacement associated with each particle between two stages of deformation. The non-affine displacement exhibits exponential crossover from ballistic to diffusive behavior with respect to the cumulative deviatoric strain, indicating that in athermal granular packings, the cumulative deviatoric strain plays the role of time in thermal systems and drives effective particle diffusion. We further study the size-dependent deformation of the granular pillars by simulation, and find that different-sized pillars follow self-similar shape evolution during deformation. In addition, the yield stress of the pillars increases linearly with pillar size. Formation of transient shear lines in the pillars during deformation becomes more evident as pillar size increases. The width of these elementary shear bands is about twice the diameter of a particle, and does not vary with pillar size.Comment: 14 pages, 11 figure

Capturing human activity spaces: New geometries

Activity space, defined as “the local areas within which people move or travel during the course of their activities during a specified time period”, is a measure of an individual’s spatial behavior which captures individual and environmental differences and offers an alternative approach to studying the spatial reach of travelers. The shape and area of activity space is a product of how it is conceptualized and measured. This paper enlarges the set of geometries which can be used to describe activity space. It tests four parametric geometries (ellipse, superellipse, Cassini oval, and bean curve), which are identified as those capturing a specific share of all locations visited, i.e. 95%, while minimizing the area covered. They are estimated for a number of long-duration data sets while distinguishing between trip purposes. We present both a flexible, easily adaptable method for calculating activity spaces of different shapes and a qualitative comparison of the four above-mentioned shape types on the basis of the given surveys. We can thus demonstrate that the choice of an appropriate shape representing an individual’s activity space is highly dependent on the spatial distributions and frequencies of the locations visited by the person in the given time period

Divergence of Voronoi Cell Anisotropy Vector: A Threshold-Free Characterization of Local Structure in Amorphous Materials

Characterizing structural inhomogeneity is an essential step in understanding the mechanical response of amorphous materials. We introduce a threshold-free measure based on the field of vectors pointing from the center of each particle to the centroid of the Voronoi cell in which the particle resides. These vectors tend to point in toward regions of high free volume and away from regions of low free volume, reminiscent of sinks and sources in a vector field. We compute the local divergence of these vectors, where positive values correspond to overpacked regions and negative values identify underpacked regions within the material. Distributions of this divergence are nearly Gaussian with zero mean, allowing for structural characterization using only the moments of the distribution. We explore how the standard deviation and skewness vary with the packing fraction for simulations of bidisperse systems and find a kink in these moments that coincides with the jamming transition

Developing a corpus of strategic conversation in The Settlers of Catan

International audienceWe describe a dialogue model and an implemented annotation scheme for a pilot corpus of annotated online chats concerning bargaining negotiations in the game The Settlers of Catan. We will use this model and data to analyze how conversations proceed in the absence of strong forms of cooperativity, where agents have diverging motives. Here we concentrate on the description of our annotation scheme for negotiation dialogues, illustrated with our pilot data, and some perspectives for future research on the issue

Linear ubiquitination at a glance

Ubiquitination (also known as ubiquitylation) is a post-translational modification that creates versatility in cell signalling and regulates a multitude of cellular processes. Its versatility lies in the capacity to form eight different inter-ubiquitin linkages through the seven lysine residues of ubiquitin and through its N-terminal methionine (M1). The latter, referred to as linear or M1 linkage, is created by the linear ubiquitin chain assembly complex (LUBAC), the only E3 ligase known to date that is capable of forming linear ubiquitin chains de novo Linear ubiquitin chains are crucial modulators of innate and adaptive immune responses, and act by regulating inflammatory and cell death signalling. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge on the role of LUBAC and linear ubiquitination in immune signalling and human physiology. We specifically focus on the role for LUBAC in signalling that is induced by the cytokine tumour necrosis factor (TNF) and its role in inflammation, gene activation and cell death. Furthermore, we highlight the roles of deubiquitinases (DUBs) that cleave M1 linkages and add an additional layer in the control of LUBAC-mediated immune signalling